1. Field of the Invention
The present invention relates to a driving method for an ink ejection device.
2. Description of the Prior Art
Of non-impact type printing devices which have recently taken the place of conventional impact type printing devices and have greatly propagated in the market, ink-ejecting type printing devices have been known as being operated on the simplest principle and as being effectively used to easily perform multi-gradation and coloration. Of these devices, a drop-on-demand type for ejecting only ink droplets which are used for printing has rapidly propagated because of its excellent ejection efficiency and low running cost.
The drop-on-demand types are representatively known as a Kyser type, as disclosed in U.S. Pat. No. 3,946,398, or as a thermal ejecting type, as disclosed in U.S. Pat. No. 4,723,129. The former, or Kyser type, is difficult to design in a compact size. The latter, the thermal ejecting type, requires the ink to have a heat-resistance property because the ink is heated at a high temperature. Accordingly, these devices have significant problems.
A shear mode type printer, as disclosed in U.S. Pat. No. 4,879,568, has been proposed as a new type to simultaneously solve the above disadvantages.
As shown in FIGS. 7(a) and 7(b), the shear mode type ink ejection device 600 comprises a bottom wall 601, a ceiling wall 602 and a shear mode actuator wall 603 disposed therebetween. The actuator wall 603 comprises a lower wall 607 which is adhesively attached to the bottom wall 601 and polarized in the direction indicated by an arrow 611, and an upper wall 605 which is adhesively attached to the ceiling wall 602 and polarized in the direction indicated by an arrow 609. An ink channel 613 is formed between two adjacent actuator walls 603. A space 615 is formed between next two adjacent actuator walls 603 so that the space 615, which is narrower than the ink channel 613, is formed next to the ink channel 613. In this manner, the ink channel 613 and the space 615 are alternately formed in the widthwise direction of the bottom wall 601 or the ceiling wall 602.
A nozzle plate 617 is fixedly secured to one end of the ink channels 613. The nozzle plate 617 is formed with nozzles 618 so as to positionally correspond to the ink channels 613. An electrode 619 is formed in one side of each actuator wall 603 and an electrode 621 is formed in the other side of the actuator wall 603. Each of the electrodes 619, 621 is formed from a metal. To insulate the metal from the ink, the metal is covered with an insulating material (not shown). The electrodes 619 which face the spaces 615 are connected to ground 623. The electrodes 621 which are provided in the inner side of the ink channel 613 are connected to a silicon chip operating as an actuator driving circuit 625.
Next, a manufacturing method for the ink ejection device 600 as described above will be described. First, a piezoelectric ceramic layer, which is polarized in a direction as indicated by an arrow 611, is adhesively attached to the bottom wall 601 and a piezoelectric ceramic layer, which is polarized in a direction as indicated by an arrow 609, is adhesively attached to the ceiling wall 602. The thickness of the piezoelectric ceramic layer to be attached to the bottom wall 601 and the ceiling wall 602 is equal to the height of the lower walls 607 and the upper walls 605. Subsequently, parallel grooves are formed to the piezoelectric ceramic layers using a diamond cutting disc or the like to form the lower walls 607 and the upper walls 605. Then, the electrodes 619 and 621 are deposited on the side surfaces of the lower walls 607 by a vacuum-deposition method, and the insulating layer is deposited onto the electrodes 619 and 621. Likewise, the electrodes 619 and 621 are deposited on the side surfaces of the upper walls 605 and the insulating layer is deposited on the electrodes 619 and 621.
The vertex portions of the upper walls 605 and the lower walls 607 are adhesively attached to one another to form the ink channels 613 and the spaces 615. Next, the nozzle plate 617 formed with the nozzles 618 therein is adhesively attached to one end of the ink channels 613 and the spaces 615 so that the nozzles 618 positionally correspond to the ink channels 613. The electrode 621 and 619 are connected to the actuator driving circuit 625 and the ground 623, respectively, through the other end of the ink channels 613 and the spaces 615.
A voltage is applied to the electrodes 621 of each ink channel 613 from the actuator driving circuit 625, whereby the actuator walls 603 defining that ink channel 613 suffer a piezoelectric shear mode deflection in such a direction that the volume of the ink channel 613 increases. For example, as shown in FIG. 8, when a voltage V is applied to the electrodes 621c of the ink channel 613c, an electric field is generated in the actuator wall 603e in the direction indicated by arrows 631 and 629 and an electric field is generated in the actuator wall 603f in the direction indicated by arrows 632 and 630. Because the electric field directions are at right angles to the polarization directions 609 and 611, the actuator walls 603e and 603f deform outward to increase the volume of the ink channel 613c by the piezoelectric shear effect, resulting in a decrease in the pressure in the ink chamber 613c. The negative pressure is maintained for a duration of time a T corresponding to a duration of time during which time pressure wave propagates one way lengthwise in the ink channel 613.
During the time duration T, ink is supplied from a manifold (not shown). The duration of time T is necessary for a pressure wave to propagate across the lengthwise direction of the ink channel. The duration of time T is given by L/a wherein L is the length of the ink channel 613 and a is the speed of sound through the ink filling channel 613. Theories on pressure wave propagation teach that at the moment the duration of time L/a elapses after the rising edge of voltage, the pressure in the ink channel 613 inverts to a positive pressure. The voltage applied to the electrode 621c of the ink channel 613c is returned to 0 volt in synchronization with the timing when the pressure in the ink channel 613 is inverted so that the actuator walls 603e, 603f revert to their initial shape shown in FIG. 7(a).
The pressure generated when the actuator walls 603e, 603f return to their initial shape is added to the inverted positive pressure so that a relatively high pressure is generated in the ink channel 613c. This relatively high pressure ejects an ink droplet from the nozzle 618c. The ink droplet thus ejected impinges upon a recording medium (not shown) spaced, for example, 2 mm, from the nozzle, thereby forming a print dot on the recording medium.
With the conventional driving method of the ink ejection device, it has been unable to adjust the diameter of a print dot to be recorded on the recording medium, because the size of the print dot is determined depending upon the recording medium, ink, the size of ink droplet ejected from the nozzle, and an ink ejection speed. If desirable size of print dot cannot be obtained, a high quality printing cannot be achieved.